1. Field of the Invention
The embodiments herein generally relate to microelectronic systems, and more particularly, to radio frequency (RF) microelectromechanical systems (MEMS) and piezoelectric MEMS actuation technology.
2. Description of the Related Art
MEMS devices are micro-dimensioned machines manufactured by typical integrated circuit (IC) fabrication techniques. The relatively small size of MEMS devices allows for the production of high speed, low power, and high reliability mechanisms. The fabrication techniques also allow for low cost mass production. MEMS devices typically include both electrical and mechanical components, but may also contain optical, chemical, and biomedical elements.
There are a number of actuation and sensing technologies used in MEMS technology; the most common are electrostatic, electrothermal, magnetic, piezoelectric, piezoresistive, and shape memory alloy technologies. Of these, electrostatic MEMS are generally the most common due to its simplicity of fabrication and inherent electromechanical capabilities. However, piezoelectric MEMS tend to out-perform electrostatic MEMS actuators in out-of-plane (vertical) displacements in terms of attainable range, power consumption, and voltage level. Parallel plate electrostatic actuators which are typical electrostatic out-of-plane actuators, generally attain vertical displacements on the order of a few microns for several tens of volts while consuming microwatts of power.
The MEMS industry has described the possibility of using piezoelectric thin films for use as microrelays or as RF MEMS switch actuators. One such microrelay device utilizes a sol-gel PZ0.52T0.48 (PZT) thin film actuator to close a direct current (DC) contact. In other conventional designs, a d33 mode of operation as opposed to a d31 mode of actuation is used.
Other conventional approaches utilize RF switches using PZT thin film actuators. Here, similar to the microrelay designs, the focus is on a cantilever structure. Moreover, some approaches use a cantilever that is perpendicular to the wave propagation direction along the RF conductor of the co-planar waveguide (CPW). Because of the relatively high dielectric constant of the PZT actuator, the RF fields can easily couple to the actuator forming a resonant structure. When the perpendicular actuator is exactly one quarter wavelength, the open circuit of the actuator will appear as a virtual ground at the center of the CPW structure causing the device to isolate the input from the output even when the switch is closed for a series switch or open for a shunt switch. If the actuator is arranged to be parallel to the CPW axis, the added capacitance of the actuator can be absorbed in the CPW itself, and no standing wave is generated as is the case for the perpendicular actuator. The result of this approach is that the switch has a better performance over a wide frequency band.
Generally, conventional MEMS switches have not been satisfactory for higher frequencies because of impedance miss-match problems and other perturbations to RF propagation. The conventional designs also typically require relatively large actuation voltages for proper functioning, and generally, the conventional designs have poor device lifetimes. Therefore, there remains a need for a RF MEMS switch capable of functioning under low actuation voltages and which provides for an increased device lifetime.